The half life of a radioactive sample is the time in which
Correct answer: B. half the undecayed nuclei present decay
- A. the whole sample decays
- B. half the undecayed nuclei present decay
- C. the activity becomes zero
- D. the mass of the sample halves
Explanation
Decay is a random process with a constant probability per nucleus per unit time, so a fixed fraction rather than a fixed number decays in each interval, which is what makes the half life constant. After two half lives one quarter remains, after three one eighth, and the count never quite reaches zero. The mass of the sample barely changes, since the daughter nuclei are almost as heavy.
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About Half-life and Rate of Decay
Half-life is the time required for half the unstable nuclei in a sample to decay, and it remains constant for a particular radioactive nuclide. Questions use the exponential decay law, decay constant, activity, remaining quantity and mean life. Radioactive decay is random for individual nuclei but predictable for large samples.
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